Categories and sub categories can be discussed as "virtual folders" and publications as files allocated inside this folders. So, to establish publications you are first to establish categories and/or sub categories for them.
Representation of publications in Realm CMS web site (using automated menus) is fundamentally based on their assignining to a category or sub category. Clicking name of category or sub category inside menu visitor of web site can view list of publications inside clicked category, - also, - if category contains some sub categories visitor can see also this sub categories.
Clicking name of category or sub category inside menus, visitor of your web site vould see list of publications inside this category (title and resume). If category contains multiple publications, - paged list vould be shown, - Realm CMS can server thousand publications inside category.
If category or sub category contains ONLY ONE publication, - clicking category name inside menu visitor of your web site wold be redirected to publication (viewing of list of publications would be avoided). This means that categories containing ONLY ONE publication are same time serving as "virtual titles", "virtual links" to publication.
Some categories and/or sub categories can be marked as "hidden" - publications inside this categories are not visible for common visitors of your web site. Hidden categories can hold publications which are just editing and are not ready to be presented to public.
Sub categories can be moved to other root catgeory or turned to root category - all publications inside this category also would be moved with it.